Dear S.,
CONGRATULATIONS! Being a Mom of 6 is great! I am a Mom of 9, 4 boys and 5 girls. One of my "boys" is 6'7". (I am 5'7", his Dad is 5'11")
I was always a head taller in my class as well. It was very difficult to find true friends and find the right clothing to wear. Try to encourage her choices to be modest regardless if she were slim. The girls I see today are dressed like little women. There is a balance between style and modesty. You can find it and it will help her to feel more confident to have you help her choose her clothing with responsibility.
What I have found when my daughters started becoming aware of their body's imperfections is to encourage better food choices and more fun exercise. I managed to sew for the ballet company for lessons. They loved ballet and it helped their body image as well as their grace. I found that focusing on their "problem" just increased their concern over how they were developing. I would tell them that they will want to make better choices and to eat better and it would all work out.
She will need lots of reassurance from you as the time progresses and while you have tried all that you can with regards to the school, you may consider homeschooling. From what I have seen from the 3-4 grade up the "school influence" just gets worse- especially with girls. (sorry)
Your Daughter sounds wonderful. I would encourage you to look elsewhere for friends for her as well. Homeschool groups usually have interesting and neat kids and church affiliations can help, though the children's attitudes are often an outcropping of the school system too.
As far as her largeness. My son, 21, has found a wonderful Fiance, 20, that is 6'2". She is lovely. It has taken a long time for anyone to realize how incredible she is, but we all know she is a choice young woman. What her size has done for her is made her very humble. She has become very studious and kind to others. She is also a runner and loves to run long distances 6 miles is usual for her.(She looks petite next to my son!
While it is hard right now, your daughter's experiences can be turned into many assets by the right reaction from you and her Dad.
And always be her Mom. She needs your guidance and will throughout her entire life. It amazes me that my older children sometimes need more reassurance now than they did as children. Be the example to her she needs.
Best Wishes,
H. B. MOM, mother of many!